This invention relates to a process for forming high resolution conducting patterns on an insulating substrate.
In the manufacture of certain electronic devices, it is necessary to provide a large number of circuit paths within a severely limited substrate area. This problem is encountered, for example, in making alphanumeric displays of the gas discharge type. In their most common form, displays of this type exhibit one or more lines of characters which may form words, or display other desired information. Each character is formed by a dot matrix array. During operation, the array is selectively energized so that the dots form the desired character.
Typically each dot matrix array comprises a matrix of thirty-five dots. A display may have forty or more such arrays, and the corresponding dots of each array are interconnected by a circuit path called an "address line." Thus, in addition to the requirements of other constituents of the display (there are, for example, forty anode electrodes), no less than thirty-five address lines are required for the dot elements. These lines must be grouped in an extremely crowded arrangement as dictated by the space constraints associated with a commercially acceptable display.
As in the case of certain other electronic circuits, it is essential that the address lines in a gas discharge display be highly conductive. In addition, the lines must be physically well defined to enable both the line width and the spacing between lines to be minimized. By so doing, a higher level of circuit density can be achieved, thereby accommodating a large number of address lines within a limited substrate area.
Commercial producers of thick film circuitry rely almost without exception upon screen printing for forming circuit paths on insulating substrates. It is because screen printing is efficient, easy to automate and therefore economical, that it has been almost universally adopted. Screen printing, however, cannot be reliably employed to produce line widths and line spacing smaller than 0.005 inches, as this is the practical limit of its resolution.
As a way of increasing the finite number of circuit paths that could be carried by a given area of a substrate, manufacturers have resorted to using multi-layer thick film structures. In such a structure, the circuit paths of a lower layer are isolated from the circuit paths of a higher layer by an intermediate dielectric layer. Since the fabrication of multi-layer structures entails multiple screening and firing operations, the manufacturing cost is for some applications considered prohibitive.
As to the use of other less traditional thick film metallization systems in critical applications such as have been described, none have been found to be capable of providing the desired line resolution at moderate cost. In some systems, high pattern resolution is achieved by the use of complex etching processes, which tend to be costly. In other systems, either the firing temperatures used are above the softening temperature of a glass substrate and therefore unsuitable, or the metallization used is a precious metal, rather than a base metal and therefore costly.
Thin film systems employing sputter or vapor deposition of circuit paths are likewise unsuited for applications such as the one considered here. These systems are comparatively inefficient, in that they are batch, rather than continuous fabrication processes. The thin film systems have the additional disadvantage of being capital intensive. That is to say, the implementation of such a system entails considerably greater equipment expense than the other systems which have been discussed.
It will be apparent, therefore, that there is a need for an economical process to form conductive patterns on glass substrates in accordance with the requirements which have been herein set forth. The present invention fulfills the need for an improvement in the metallization art by providing an economical process by which high resolution circuit patterns can be formed on substrates with the use of relatively low firing temperatures.